1. Field of the Invention
The present invention relates to a boundary microphone that is mainly placed on a table upon use, and more specifically to a boundary microphone having a unit with which a sound pickup direction can be easily changed.
2. Description of the Related Art
Boundary microphones are known that are mainly placed on a table upon use. FIG. 11 illustrates an example of a conventional boundary microphone. A base 27 of the boundary microphone illustrated in FIG. 11 is formed of a flat piece of metal and can be fixed on a surface of a desk or a floor. In an upper surface of the base 27, a cavity (dimple) for mounting a circuit board 28 and the like is formed. From the upper surface of the base 27, a column 21 integrally formed with the base 27 is erected, approximately in the center when viewing the base 27 from the plane direction. A later described cover 20 is connected to the base 27 via the column 21. In the column 21, a tapped hole 22 is formed from the upper end. For the base 27, the left side and the right side in FIG. 11 are the front side and the backside, respectively, and a wall is integrally formed at the back end of the base 27. A bush 24 is fitted into a hole that is formed through this wall. In the case of a boundary microphone installed in a conference room, the boundary microphone is installed on a table or the like with the front side facing the participants. One end of a microphone cord 25 is passed through a center hole of the bush 24. The microphone cord 25 is generally a cord constituted of a two-core balanced output cord and a shielding wire wrapping around the two-core balanced output cord. The end portions of the above-described two-core signal cable and shielding wire constituting the microphone cord 25 are connected to a predetermined soldering land or the like on the later-described circuit board 28.
In the upper surface of the base 27, the circuit board 28 is secured to the approximately front half portion of the above-described cavity so as to plug the cavity. In the upper surface of the circuit board 28, a microphone unit 26 is mounted with the sound wave introducing port thereof facing forward. As the microphone unit 26, a capacitor microphone unit is generally used. The cover 20 for covering the whole upper surface of the base 27 including the microphone unit 26 and the circuit board 28 is put on the base 27. The cover 20 is made of a metallic material, as in the case of the base 27, and numerous openings for introducing sound waves to the microphone unit 26 are formed therein. As the material of the cover 20, generally, a punching metal is used, in which numerous holes are formed by punching. The cover 20 is press molded into a flat plate form which is then inverted and put on the upper surface of the base 27. In the cover 20, approximately in the center when viewing the cover 20 from the plane direction, a dimple is formed at a position corresponding to the column 21 of the base 27 and a hole is formed in the bottom of this dimple. A screw 23 as a fastening member is inserted into this hole, and the screw 23 is screwed into the tapped hole 22 formed in the column 21, so that the cover 20 is fastened to the base 27. The head of the screw 23 sinks into the inside of the dimple of the cover 20. A receiving portion for the peripheral portion of the cover 20 is formed in the periphery of the upper surface side of the base 27, and the receiving portion is designed so that the peripheral portion of the cover 20 can be in contact with the base 27 while the cover 20 is fastened to the base 27 with the screw 23 as described above.
As described above, the boundary microphone includes mainly two parts, i.e., the base 27 and the cover 20 in appearance, and the internal components are incorporated in the internal space (see, for example, an invention disclosed in Japanese Patent Application Publication 2009-100157). The screw 23 inserted in the hole of the cover 20 is screwed into the tapped hole 22 of the base 27, and whereby the base 27 and the cover 20 are mutually fastened. The base 27 and the cover 20 are fastened together with one screw 23 approximately in the center of the cover 20 and the head of the screw 23 fits into the dimple of the cover 20.
A conventional boundary microphone having a circular shape as illustrated in FIGS. 12 and 13 is available. In FIGS. 12 and 13, the circular boundary microphone mainly includes: a base 117 having a circular planer shape; a cover 120 having a shape of a lid with a circular planer shape; microphone units 116 installed on the base 117 and covered with the cover 120; a cord insertion hole 125 provided at a center position of the base 117 as viewed from the plane direction, through which an internal space covered by the cover 120 is communicated with the outside; an external connection cord 123 inserted through the cord insertion hole 125; a rubber plate 122 provided under the base 117; and a tubular shaft 124 fitted in the cord insertion hole 125.
The cord insertion hole 125 is formed on the base 117. The internal space covered by the cover 120 is communicated with the outside through the cord insertion hole 125. The cord insertion hole 125 is formed at the intersection point of the central axis lines of the three microphone units 116 that are radially arranged. The base 117 is made of a flat piece of metal and the rubber plate 122 provided thereunder serves as a supporting base therefor. The cover 120 is pressed by a pressing material 121 in the inner diameter direction thereof. An insertion hole for a screw 113 is provided at the center of the cover 120 as viewed from the plane direction. The cover 120 is fixed to the base 117 with the screw 113 being screwed into a tapped hole appropriately provided on the tubular shaft 124 through the insertion hole. A circuit board not illustrated is provided on the upper surface of the base 117.
The above described boundary microphone includes mainly three parts, i.e., the base 117, the cover 120, and the pressing material 121 in appearance, and the internal components are incorporated in the internal space. The boundary microphone needs to be adjusted to have desired sound pickup direction upon use. More specifically, installation direction of a microphone unit 116 incorporated in a boundary microphone is changed by opening the cover 120 and manually dismounting the microphone units 116, moving the microphone units 116 to the desired directions, and fixing the microphone units 116 thereat.
The above described process takes time and has to be performed by a skilled person to prevent failure inside the boundary microphone. Thus, maintenance cost is required. A person with certain knowledge may perform the maintenance. However, in such a case, the resistance against external noise may not be ensured after the maintenance because the maintenance involves processing on internal structure that is related to radio-frequency interference (RFI) resistance.